1. Field of the Invention
This invention relates generally to instruments for use in navigating, and more particularly to automatic devices for determining and pictorially displaying the position and related dynamic parameters of a traveling craft--and for guiding or helping to guide such a craft upon a straight track to desired destinations or way points, practically independent of current.
2. Prior Art
One of the earliest sophisticated systems for automatically producing a pictorial representation of craft position is disclosed in U.S. Pat. No. 1,701,582 to Georges Mengden. Mengden's paper map was presented directly in a movable case, with a wind-up mechanism for strip maps covering the anticipated terrain of a long journey. Input information was derived by cable drives from a compass repeater and from a speed sensor--namely, a ship's "speed log" in the case of a water craft, anemometer in the case of an airplane, or odometer in the case of a land vehicle.
Mengden relied upon an ingenious mechanical system--a rubber ball suspended between two drive rollers and four driven rollers, with axis of rotation of the drive rollers controlled by the compass repeater--to resolve the scaled-down craft motion into cartesian components to drive the map in the "up and down" and "side to side" directions respectively. For water and air craft Mengden provided drift correction in the form of a duplicate motion-resolving system, complete with six rollers and rubber ball, in which the drive rollers were oriented and motor-driven at appropriate angle and velocity to approximate the known set and drift of the medium through which the craft was moving. These parameters of course had to be determined or estimated by the operator and set up at the controls of the device.
Such analog mechanical systems received extensive elaboration. U.S. Pat. No. 3,160,851 to Karel Ramsayer was addressed to the problem of limited area coverage of maps in devices such as Mengden's, and the inherent difficulty of using strip maps of a preselected itinerary for general navigation. Thus Ramsayer's device "presupposes the employment of a navigation computer delivering the map track components as the angle of rotation of two output shafts (see e.g. W. H. Coulthard: Aircraft Instrument Design, p. 163, Sir Isaac Pitman and Sons, Ltd., London." Ramsayer prepared multiple map strips, covering adjacent portions of a conventionally shaped navigational chart, but with overlap areas along both edges. Such strips he connected end-to-end to make a roll, and the roll was mounted on and driven by platens within his mechanism.
Ramsayer's platen drive mechanism responded to two different kinds of situations: (1) it operated at speed proportional to north-south components of craft motion, to reproduce such components as motion of the map strip past a horizontal hairline; (2) it stepped from one strip to the next, at a rapid slewing rate, to accommodate motion of the craft past the left- or right-hand (that is to say, western or eastern) edges of a particular map strip. Meanwhile a traveling spot of light (or a traveling pen) operated from left to right to pinpoint the present east-west position of the craft on the map strip currently displayed. An analog electronic limit system used master and slave potentiometers, and/or stepping switches with fixed resistances, to respond to motion of the light spot or pen into the overlap areas along the edges, by initiating a map-strip change to keep the right map sections on display.
To minimize the problem of continuous map shifts when the craft moved in a meandering course that generally followed the edge of the displayed region, the permissible incursion of light spot or pen into the map overlap areas was adjustable. A small-scale map was interspersed into the roll of strip maps every so many panels, to permit the navigator to readily call up a view of a larger area; when these particular map strips were displayed the apparatus automatically switched-in a correspondingly different set of scale-determining potentiometers.
While Ramsayer directed his efforts to minimizing the problems of navigators whose craft were to range over a wide area along unpredicted routes, others addressed the opposite problem of the relatively small scale available when using even large-scale maps, in systems such as Mengden's and Ramsayer's. U.S. Pat. No. 3,725,919 to Jerry Jones reflects such a concern, in the context of a pipelaying barge, whose mission includes adhering as closely as possible (in the range of feet or yards) to a preselected route, and producing a precise record (in the range of feet or yards) of the deviation from that route, for future purposes of pipeline maintenance.
Since the preselected route is not generally straight north-and-south, the Ramsayer system is not readily adapted to provide very high-resolution display of such a route, and in the Ramsayer system the resultant record would in general consist of pen markings spanning several map strips. Worse yet, since the preselected route is not generally straight at all, even a strip map specially prepared to show the route would have to be wide enough to accommodate the entire curvature of the route and thus would be severely limited in resolution. Jones' solution to this problem was to dissect the course into plural segments, each at least nearly rectilinear, and to lay out each segment of the course along the center of a long, narrow chart segment. Jones' chart strips, unlike Mengden's and Ramsayer's, were not aligned north-and-south (i.e., parallel to meridians), but rather each employed grid bearings different, in general, from those of the previous or subsequent segment.
Jones' system is more modern than Ramsayer's in that Jones employs a radio ranging system--two shore-based transponders interacting with a shipboard transceiver/computer unit--and also in that Jones uses a general-purpose digital computer (such as the Hewlett Packard Model No. 2115A) to determine the coordinates of the barge position, and from these coordinates then to generate control signals for stepping motors that drive the chart platen and pen. Jones' system, however, is limited to the presentation of position on specially prepared paper maps. It is further limited by the use of a general-purpose computer, complete with peripheral devices (card, tape, or the like). It thus by size, power requirements, and necessary operating skill is unsuited for operation in general navigation by nontechnically trained personnel aboard small craft. Moreover, by its use of a single computer for all functions, it requires a computer of unnecessarily great computing capability and therefore inappropriately high price.
Perhaps the most striking limitation of Jones' system is its restriction to near-shore areas, a restriction that arises from its dependence upon dedicated shore-based transponders. The Jones system, however, also has a less apparent limitation in that it is not designed to make or use dead-reckoning calculations. Although it is perhaps conventional wisdom to think of radio-fix technology as more sophisticated, more powerful and more modern than dead-reckoning technology, there is--as will be explained below--a generally unrecognized advantage that accrues from combining the two capabilities in a single navigating instrument.
Many inventions have been directed to minimizing the various difficulties of map handling and display for different applications. For instance, U.S. Pat. No. 3,299,539 to H. Leiber employs a "lost motion" effect to avoid the problem of repetitive map exchange when a craft wanders back and forth along the edge of one map. U.S. Pat. No. 2,857,234 to T. Murray discloses a plurality of maps, showing either nested areas at different scales or adjacent areas. Each map has its own indicating or recording apparatus. With this system there is relatively less delay and confusion when the indicator leaves one map and the motion continues to be indicated on another, but the additional equipment complexity and space requirements are obviously acceptable only in unusual situations.
Perhaps the most elaborate effort at solution is the closed-circuit television approach typified by U.S. Pat. No. 2,836,816 to J. Allison. An "optical computer" and amplifying components are located in a part of an aircraft remote from the cockpit, but are linked by closed-circuit television transmission to an optical display at a small cathode-ray tube in the cockpit. The automatic remote equipment comprises a reel of charts on motion-picture film, with automatic selection of frames and analog "computer" equipment for determining position and applying automatic correction for the earth's curvature.
Some workers have hit upon the idea of using a film transparency chart directly in a display unit, in the wheelhouse or cockpit, with a relatively small portion of the total transparency projected on a screen for viewing. The reported applications of this idea, however, have not been adapted for practical use in general navigation of medium-small civilian craft. For example, U.S. Pat. No. 2,814,199 to A. Waldorf discloses an air navigation system in which a strip map is projected onto a screen for comparison with a radar or optical image of nearby terrain. The navigator adjusts the equipment to bring the two superimposed images into alignment, thereby permitting both altitude determination and determination of map location. Besides requiring relatively sophisticated users and an elaborate dual display system, this system may be of marginal usefulness over open sea, or in any area where landmarks producing distinct radar images are absent.
While Waldorf's system relies upon the human operator to interpret radio signals in terms of a projected map, others have attempted to automate this interpretation and matching function. U.S. Pat. No. 3,475,754 to Royal Scovill discloses a projected-map system that relies upon two omni-bearing receivers to automatically position the map relative to the projection system. Scovill's device finds position relative to two omni-bearing transmitters of known location, by independent operation of two omni-bearing receivers, and in effect locates the craft at the intersection of two bearing lines from the two known station locations. In an alternative operating mode, Scovill's device finds position relative to one omni-bearing transmitter at a known location, by independent operation of one omni-bearing receiver in conjunction with a distance-measuring equipment receiver--thus in effect locating the craft at the intersection of a bearing line and a distance circle from the same station.
Scovill's analog system "duplicates in miniature the space relationship of the radio navigation aids, simulated by [omni-bearing] locator discs, and the vehicle, simulated by one follower related to each locator disc. . . . The instantaneous intersection of the bearing lines from the two reference omni radio stations in the simulation is sought out by [servo-controlled mechanical] followers mounted on the film carriage. Thus . . . the film is continuously driven to project the location of the vehicle at the center mark of the screen." A similar operation occurs in the mode that combines distance measurement with one bearing measurement.
Scovill's apparatus is subject to several sources of very large imprecision and inaccuracy. First, the omni-bearing signals are themselves unreliable. The bearings found in dependence upon these signals are often indeterminate to any better than plus or minus fifteen degrees--due to diffusion of the radiated field pattern by atmospheric diffraction, multiply reflection at topographic features or large buildings, and scattering from a great variety of objects (including other craft). All of these distortions are variable, in dependence upon numerous unknown factors, thus precluding meaningful efforts to compensate by showing the direction and magnitude of distortion on standard charts. The intersection of two such highly inaccurate and imprecise bearing fixes is of course little more than a stab in the dark at a very large area on the charts; yet the Scovill device would yield a deceptively authoritative-seeming map position.
Second, the interpretive electronics of the Scovill system operate entirely on an analog basis, and so are subject to variable and unknown imprecisions and nonlinearities (inaccuracies) that are not reproduced in the charts. Moreover, the charts themselves are normally Mercator, Lambert, or other projects, constructed on the basis of earth-surface dimensions as convoluted with a mathematical function of latitude. Scovill mentions (at his FIG. 13 and columns 36 through 38) applying a compensation for chart curvature due to the Lambert projection, but does not actually disclose how this might be accomplished.
Assuming that he has in mind compatible electronic systems--namely, analog systems, perhaps involving nonlinearly wound slidewires or the like--it will be apparent that such systems will be relatively inaccurate, besides being cumbersome and expensive. Moreover, since Scovill's system operates on an analog basis, overall, it would be necessary to adjust the Lambert compensators for known latitude. Yet the operator of the Scovill system presumably is reliant upon the Scovill system to determine latitude, so it would appear that some iterative operations by the operator are required to bring the system into even approximately Lambert-corrected balance.
This subject leads to a third major source of difficulty with operation of the Scovill unit, namely, the fact that latitude and longitude apparently are derived from the chart markings as a result of the readout process. This is a serious limitation, for it implies that only when the chart transport mechanism and the projection system are both operational can the operator obtain latitude and longitude readings. When any part of those subsystems is turned off, malfunctioning, or in a transitional condition (e.g., during an automatic change of charts), no latitude and longitude readings are available.
Although there is some indication in the Scovill disclosure that a separate display of latitude and longitude may be provided by counters gear-driven from the transport servomechanism, it would appear that even these counters would require at least a fully operational (and nontransitional) transport servo for operation. Further, the latitude and longitude outputs would be disabled whenever the transport was used for another purpose-such as to manually control the chart display to make preliminary observations of a portion of the charts other than that in which the craft is currently located, or to examine reference data tabulated on other frames in Scovill's film magazine. Similar limitations may be found in Scovill's provision of displayed heading and speed of craft, and bearing and distance to navigation station, way station, or destination.
Finally, Scovill's unit--like the systems of Waldorf, Jones, and others who disclose radio-fix systems--does not appear to make any provision for incorporation of dead-reckoning computation. As previously noted, this limitation forecloses enjoyment of a particularly useful mode of operation. Scovill's disclosure indicates the unfortunate consequences of this limitation, when he describes (at his column 22) the connection of his navigating device to control an autopilot: "The signals thus made available will indicate left or right off course movement of the aircraft with respect to a fixed course on the bearing line indicated to the way station or destination point with no further adjustment of bearing indication."
The Scovill system may "indicate" off-course movement, but can do so only through continued observation by the operator, who is thus implicitly placed back in the control loop, like the operator of Waldorf's system. Scovill's apparatus apparently does not read out, use, or determine the magnitude of the heading error. As will be seen, the present invention obviates this limitation.
Other radio navigation systems that are in regular use include Loran (long-range navigation) and--in short-range air navigation--two-station DME (distance-measuring equipment, similar to the single unit employed by Scovill's alternative system). Most use that is made of these navigational aids involves manual plotting of position on paper charts. Loran, for example, involves a sizable number of publicly operated radio transmitters, broadcasting position-fix signals that are separated from one another by certain time differences. Specialized receivers are used that are capable of receiving three or four such signals, comparing two of them to obtain one received time difference, and comparing another two of the signals to obtain a second received time difference.
A navigator reads these two received time-difference values from his radio equipment, and he refers to specially prepared charts on which lines of constant Loran time difference appear. By visual interpolation on his charts the navigator finds his present position as the intersection of two time-difference lines. Unfortunately, compounding the delay and inaccuracy in such plotting and interpolation, substantial inaccuracies in the charted Loran time-difference lines have been reported. Even though the Loran radio signals (and the known positions of the Loran transmitters) provide adequate information to calculate latitude and longitude directly, such calculation is generally beyond the ability of small-craft navigators, and commercially available equipment, at least, does not provide such direct readout. The charted time-difference lines offer no suggestion that correction may be required, and certainly do not hint what the direction or magnitude of such correction should be.
Two-station DME fixes are considerably more straightforward and sometimes more accurate, though they too require manual plotting on a chart. The area coverage of the DME transmitter system, however, is not as extensive as might be desired. As with Loran, the DME devices do not read out latitude and longitude directly; such position coordinates may be obtained only through the added step of manual plotting on a chart, and then reading the latitude and longitude from the chart itself.
Even these plotting procedures leave yet additional effort to be expended if the navigator wishes to compute a "course to steer." This effort may be particularly complex on long trips, for which apparently straight lines on the charts are not really straight; rhumb-line corrections are required. Moreover, even when the navigator has set his course, vigilance is required to avoid missing his destination because of motion of the medium through which his craft is moving--that is, in maritime parlance, "set and drift."
It goes without saying that each of these navigating systems serves a good purpose, within its limitation. These systems are the best that have been available--clearly far beyond the capabilities of primitive navigating systems of only fifty years ago. It is not the pupose of the foregoing comments to derogate these systems, but only to indicate certain particulars in which the present invention is believed to improve upon them.